1. Field of the Invention
The present invention relates to a multiple-access communications system which has particular, but not exclusive, applications in mobile radio dynamic channel assignment systems, local area networks (LANS) and satellite links. For convenience of description, the invention will be described with reference to mobile radio dynamic channel assignment (trunking) systems but it is to be understood that the same methods apply to other multiple-access communications systems.
2. Description of the Prior Art
Trunking systems are characterised by the problems of many users attempting to gain access to them at the same time. These attempts for access (which can be thought of as requests for service) can clash and be mutilated and in the absence of any form of control can produce an unstable situation where the number of requests for service which are mutilated increases, resulting in an increase in the number of requests which are retransmitted which, in turn, leads to further mutilation and ultimately results in a complete blockage of the system. The requests for service are transmitted to a central system control computer, hereinafter referred to as a system controller, via a signalling channel and the system controller allocates the speech channels according to some predetermined criteria. In the simplest case of a single channel system then the single channel has to be used for signalling and speech.
In order to mitigate these problems of clashing, controlled multi-access protocols are used to discipline users trying to gain access. Also the throughput, that is the number of successfully serviced requests per unit time, of the system can be increased.
N. Abramson "The Aloha System--Another Alternative for Computer Communications" AFIPS Conference Proceedings 1970 Fall Joint Computer Conference, 37, 281-285 proposed one of the first multi-access protocols termed "Pure Aloha". With this protocol, users transmit a request and wait for some form of acknowledgement of their request from the system controller; if no acknowledgement is heard users wait a random time before re-trying.
The throughput of "Pure Aloha" was doubled by a modified protocol, termed "Slotted Aloha", which allows users to transmit requests only within discrete timeslots, each request occupying one time slot. In spite of this improved throughput, "Slotted Aloha" nevertheless has practical disadvantages for example instability during busy periods.
With the objective of overcoming these disadvantages, "Slotted Aloha" was extended by a protocol, termed "Framed Aloha", which is disclosed in British Patent Specification No. 2063011A. In Framed Aloha a synchronisation invitation message, termed "Aloha Now", is transmitted by the system controller on the signalling channel at intervals indicating that the immediately following n time slots are available for users to transmit requests (either new requests or re-transmissions after unsuccessful requests) to the system controller via the signalling channel. The number of time slots n is a constant determined at the system design stage. With this protocol the requests are contained within known time frames, simplifying the system control strategy. However a drawback to having a fixed number n of time slots is that it does not take into account the variation in the number of requests between a quiet period and a busy period and this can result in unnecessarily long message delays during quiet periods and instability during busy periods.
An attempt to match the number of time slots available with the number of requests is disclosed in British Patent Specification No. 2069799B and in the corresponding U.S. Pat. No. 4,398,289, issued Aug. 9, 1983, and is termed "Dynamic Frame Length Aloha" (DFLA). This protocol includes means for dynamically controlling the frame-length, that is updating the number n of time slots on a frame-by-frame basis. The number n is calculated by observing the events in the previous frame such as the number of garbled (or clashed), empty and successful slots and from an estimate of the call arrival rate. By using feedback control in this way stability can be achieved under many operating conditions and additionally the access time (the time delay between a user wishing to make a request, and the request being acknowledged) is reduced. However this form of DFLA can only be stable provided the frame-length can be increased indefinitely to cope with very heavy demands for requests for service. In practice this is not possible because the Aloha message contains only a finite number of data bits to specify the different frame-lengths and therefore it follows that the throughput of DFLA can be low under heavy traffic loading. In addition there are circumstances where very long frames are undesirable. In the opposie situation under light traffic DFLA has the disadvantage that in order to minimise access times the central base station transmitter normally transmits continuously. This increases the likelihood of interference to other radio systems and also may reduce the working life of the transmitter.
Another approach to provide stability under conditions of heavy traffic is disclosed by John I. Capetanakis in "Tree Algorithms for Packet Broadcast Channels", IEEE Transactions on Information Theory, Vol. II-25, No. 5, September 1979 pages 505 to 515. In the tree algorithm, a tree comprises a root node from which a pair of branches extend. Each of the said branches divides into two at respective nodes and subsequent divisions by two take place at further nodes until one reaches the situation of a pair of sources being connected by respective minor branches to an associated node. In an example given, each of the branches from the root node are treated as two rooted subtrees. Signalling is carried out in pairs of slots, wherein each said slot has a width equal to a packet which is formed by a fixed length block of digital data. In operation each of the rooted subtrees is invited to send in requests for service in its respective slot of the pair of slots. If a collision/garbling is detected then the system resolves the contention before issuing another general invitation for service. When resolving contentions, one of the two rooted subtrees is considered and the contention(s) is (or are) resolved before the other of the rooted subtrees is considered. The disadvantages of this type of tree algorithm are that an algorithm which deals sequentially with distinct sub-trees may be too complex for practical implementation. Further if at one node there are two requests for service, one much stronger than the other, the weaker one may be overlooked due to capturing of an FM channel. Finally by allowing only one slot for reply then if there is contention, a further division or subset has to be considered and in consequence time is lost through additional signalling.